As for the birds his pages are full of references to them and all of his bird similies are couched in terms that show how sympathetically observant he was of their habits and ways. He knows their different methods of flying in groups and singly, he has observed them on their nests and knows their wonderful maternal anxiety for their young, and describes it with a vividness that would do credit to a naturalist of the modern time who had made his home in the woods. Indeed some of his figures taken from birds constitute examples of the finest passages of poetic description of living nature that have ever been written. The domestic animals, moreover, especially the cat and the dog, come in for their share of this sympathetic observance, and he is able to add greatly to the vividness of the pictures he paints by his references to the well-known habits of these animals. It is no wonder that the tradition has grown up that he was fond of such pets and possessed several of them that were well-known to the early commentators on his poems, and the subject of no little erudition. Nothing escaped the attention of this acute observer in the world around him, and over and over again one finds surprising bits of observation with regard to natural phenomena usually supposed to be quite out of the range of the interest of {315} medieval students generally, and above all of literary men of this Middle Age. Alexander Von Humboldt calls attention in a well-known passage in his Cosmos to the wonderful description of the River of Light in the Thirtieth Canto of the Paradiso. "I saw a glory like a stream flow by. In brightness rushing and on either shore Were banks that with spring's wondrous hues might vie. And from that river living sparks did soar, And sank on all sides in the flow'rets' bloom, Like precious rubies set in golden ore. Then, as if drunk with all the rich perfume, Back to the wondrous torrent did they roll, And as one sank another filled its room." Humboldt explains this as follows, with a suggestion that deserves to be remembered. "It would almost seem as if this picture had its origin in the poet's recollection of that peculiar and rare phosphorescent condition of the ocean in which luminous points appear to rise from the breaking waves, and, spreading themselves over the surface of the waters, convert the liquid plain into a moving sea of sparkling stars."